Today, various display devices have been under active study and development, in particular those based on electroluminescence (EL) from organic materials.
In the contrast to photoluminescence, i.e. the light emission from an active material as a consequence of optical absorption and relaxation by radiative decay of an excited state, electroluminescence (EL) is a non-thermal generation of light resulting from the application of an electric field to a substrate. In this latter case, excitation is accomplished by recombination of charge carriers of contrary signs (electrons and holes) injected into an organic semiconductor in the presence of an external circuit.
A simple prototype of an organic light-emitting diode (OLED), i.e. a single layer OLED, is typically composed of a thin film of the active organic material which is sandwiched between to electrodes, one of which needs to be semitransparent in order to observe light emission from the organic layer; usually an indium tin oxide (ITO)-coated glass substrate is used as anode.
If an external voltage is applied to the to electrodes, charge carriers, i.e. holes, at the anode and electrons at the cathode are injected to the organic layer beyond a specific threshold voltage depending on the organic material applied. In the presence of an electric field charge carriers move through the active layer and are non-radiatively discharged when they reach the oppositely charged electrode. However, if a hole and an electron encounter one another while drifting through the organic layer, excited singlet (anti-symmetric) and triplet (symmetric) states, so-called excitons, are formed. Light is thus generated in the organic material from the decay of molecular excited states (or excitons). For every three triplet excitons that are formed by electrical excitation in an OLED, only one symmetric state (singlet) exciton is created.
Many organic materials exhibit fluorescence (i.e. luminescence from a symmetry-allowed process) from singlet excitons: since this process occurs between states of same symmetry it may be very efficient. On the contrary, if the symmetry of an exciton is different from the one of the ground state, then the radiative relaxation of the exciton is disallowed and luminescence will be slow and inefficient. Because the ground state is usually anti-symmetric, decay from a triplet breaks symmetry: the process is thus disallowed and efficiency of EL is very low. Thus the energy contained in the triplet states is mostly wasted.
Luminescence from a symmetry-disallowed process is known as phosphorescence. Characteristically, phosphorescence may persist for up to several seconds after excitation due to the low probability of the transition, in contrast to fluorescence which originates in the rapid decay.
However, only a few organic materials have been identified which show efficient room temperature phosphorescence from triplets.
Successful utilization of phosphorescent materials holds enormous promises for organic electroluminescent devices. For example, an advantage of utilizing phosphorescent materials is that all excitons (formed by combination of holes and electrons in an EL), which are (in part) triplet-based in phosphorescent devices, may participate in energy transfer and luminescence. This can be achieved either via phosphorescence emission itself, or using phosphorescent materials for improving efficiency of the fluorescence process as a phosphorescent host or a dopant in a fluorescent guest, with phosphorescence from a triplet state of the host enabling energy transfer from a triplet state of the host to a singlet state of the guest.
In each case, it is important that the light emitting material provides electroluminescence emission in a relatively narrow band centered near selected spectral regions, which correspond to one of the three primary colours, red, green and blue, so that they may be used as a coloured layer in an OLED.
As a means for improving the properties of light-emitting devices, there has been reported a green light-emitting device utilizing the emission from ortho-metalated iridium complex
Non Patent Citation 0001: (Ir(ppy)3: tris-ortho-metalated complex of iridium (III) with 2-phenylpyridine (ppy). Appl. phys. lett., 1999 vol. 75, p. 4. ISSN 0003-6951.
Thus,
Non Patent Citation 0002: LEE, Chang-Lyoul. Polymer-based blue electrophosphorescent light-emitting diodes using a bisorthometalated Ir(III) complex as triplet emitter. Chemistry of Materials, 2004 vol. 16, no. 23, p. 4642-4646.
discloses bis-orthometalated Iridium complexes bearing as ancillary ligands a combination of a cyano anion with a monodentate phosphorous donor, like notably Ir(ppy)2 P(n-Bu)3CN complex (ppy=2-phenylpyridine), whose structure is depicted here below:
Non Patent Citation 0003: NAZEERUDDIN, Md. K. Highly phosphorescent iridium complexes and their application in organic light-emitting devices. J. Am. Chem. Soc., 2003 vol. 125, no. 29, p. 8790-8797. ISSN 0002-7863.discloses anionic mixed ligand Iridium (+111) complexes obtained by reaction of [IR(ppy)2(Cl)]2 (ppy=2-phenylpyridine) with tetraalkylammonium cyanide, thiocyanate or cyanate, whose structures are depicted here below, exhibiting high phosphorescence quantum yields:
Patent Citation 0001: U.S. Pat. No. 6,245,988 B (ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE). 2001 Jun. 12.discloses photosensitizer complexes useful in photovoltaic cells complying with formulae (Ia) and (Ib) here below:MX3Lt  (Ia)MXYLt  (Ib)wherein M is a transition metal chosen among Ru, Os, Fe, Re and Tc, X is a co-ligand independently selected from NCS−, Cl−, Br−, I−, CN−, NCO−, H2O, NCH2−; pyridine unsubstituted or substituted by at least one group selected from vinyl, primary, secondary or tertiary amine, OH and C1-30 alkyl, Y is a bidentate N□N ligand chosen among substituted or unsubstituted bipyridine or o-phenantroline, and L, is a tridentate N□N□N ligand comprising heterocycles such as pyridine and/or triazole.Patent Citation 0002: U.S. Pat. No. 6,670,645 B (DUPONT DE NEMOURS. 2003 Dec. 30.discloses electroluminescent Ir(III) compounds comprising substituted 2-phenylpyridines, phenylpyrimidines, and phenylquinolines and, optionally, ancillary monodentate ligands such as chloride and nitrate anions, phosphines, isonitriles, carbon monoxide, mono-amines.Patent Citation 0003: US 20050048312 A (DU PONT DE NEMOURS). 2005 May 3.discloses electroluminescent complexes comprising, inter alia, additional ligands such as permutations of amines, phosphines, alkoxydes, halides, hydrides or orthometalated aryl groups.Patent Citation 0004: WO WO 2006/012023 A (EASTMAN KODAK COMPANY). 2006 Feb. 2.discloses a process for forming a tris-cyclometallated iridium or rhodium complex comprises reacting in the presence of a solvent: a) a bis-cyclometallated complex (A) comprising an Ir (III) or Rh (III) metal, two bidentate ligands, two monodentate ligands and a counterion, and b) a heterocyclic compound capable of forming an organometallic cyclometallated complex. Among bis-cyclometallated complexes (A), mention is notably made of ionic Ir complexes bearing either nitrile ligands (cationic complexes A1 to A3 here below) or thiocyanate anions (anionic ligands A4 and A5):

However, since the foregoing light-emitting materials of the prior art do not display pure colours, i.e. their emission bands, generally limited to green, are not centered near selected spectral regions, which correspond to one of the three primary colours, red, green and blue, the range within they can be applied as OLED active compound is narrow. It has thus been desired to develop light-emitting materials capable of emitting light having other colours, especially in the blue region. Triple emissive blue has been recognized in the art as difficult to attain due to the high energy of the emissive state.
Efficient long-lived blue-light emitters with good colour coordinates are a recognized current shortfall in the field of organic electroluminescent devices.
Patent Citation 0005: US 2005/0112406 2005 May 26.
discloses organometallic complexes suitably used for formina an organic layer of an electroluminescence device, providing maximum emission in the wavelength range of 400 to 650 nm, complying with formula here below:
wherein M is a metal chosen among, inter alia, Ir; CyN represents a heterocyclic group containing nitrogen bonded to M; CyC is a carbocyclic group containing carbon bonded to M; CyN-CyC being a cyclometalating ligand bonded to M; A is a ligand containing nitrogen bonded to M; X is a monoanionic monodentate ligand chosen among inter alia, CN, SCN, OCN. Said document discloses, in particular, complexes bearing two substituted phenylpyridine ligands, an imidazoyl ligand and a cyanide, thiocyanate, or cyanate anion.Patent Citation 0006: US 2001019782 (FUJI PHOTO FILM). 2001 Sep. 6.discloses light emitting iridium complexes bearing orthometalated ligands. Among suitable complexes, mention is notably made of compound complying with formula hereinafter:
comprising tow phenylpyridine orthometalated ligands, a pyridine ligand and a cyanide anion.Patent Citation 0007: US 2002182441 (TRUSTEE OF PRINCETON UNIVERSITY). 2002 Dec. 5.discloses emissive phosphorescent organometallic compounds producing electroluminescence, particularly in the blue region of the visible spectrum. In particular said document discloses a compound of formula:
comprising two fluorine-substituted phenylpyridine ligands, a pyridine ligand and a cyanide anion.